The present invention is directed to an x-ray diagnostics installation having a transmission chain which is subject to imaging errors. The transmission chain has an x-ray image intensifier and an optical means for imaging the output image of the x-ray image intensifier onto a video pick-up means coupled to the optical means. Such apparatus serve, for example, for video reproduction of x-ray pictures.
German reference DE-A-31 27 648 discloses such an x-ray diagnostics installation wherein the output image of the x-ray image intensifier is imaged onto a coupled image pick-up means by a basic objective coupled to the x-ray image intensifier and by a camera objective. In this case, the image is produced on a target area of a video camera.
It is not only the x-ray image intensifier but also the video chain that produce pillow-shaped distortions. A non-uniform brightness distribution ("vignetting"), that drops off concentrically from the middle of the image to the edge of the image, results in the image from these distortions as well as from the distance of the camera objective from the basic objective.
In order to obtain a more uniform brightness distribution, the video signal was previously differently amplified, whereby the signal at the image edge was more amplified than that in the middle of the image. However a lower signal-to-noise ratio results at the edge of the image, so that the noise is disturbingly increased. Moreover, this method of producing a more uniform brightness distribution is only effective in a video camera. It is not effective in other image pick-up devices such as, for example, a motion picture camera or a medium format camera.